Non-volatile memory (NVM) has been widely used as computer memory that can retain the stored information even when there is no power supply. Examples of non-volatile memory include read-only memory, flash memory, and most types of magnetic computer storage devices. Programming in NVM is achieved by applying a positive programming voltage to the drain region and connecting the source region to ground, thereby charging the floating gate (FG) via channel hot electron (CHE) injection from the drain region. Therefore, the hot carrier injection efficiency determines the program speed of the memory device.
To achieve efficient hot carrier injection, high lateral and vertical electric fields are desired to cause impact ionization and attract electrons to the gate respectively. Impact ionization is a phenomenon where an energetic charge carrier loses energy by the creation of other charge carriers. In particular, in semiconductors, an electron (or hole) with enough kinetic energy can transfer its energy to an electron in the valence band, and promote it to the conduction band, hence creating an electron-hole pair. However, in a conventional metal-oxide-semiconductor field-effect transistor (MOSFET) for example, when the gate voltage is increased, the vertical electric field increases whereas the lateral electric field decreases. As a result, the MOS device has low injection efficiency.
From the foregoing discussion, it is understood that challenges exist to achieve high hot carrier injection efficiency with low power for a fast programming memory device.